This project will be to determine the motional modes of the muscle protein actin. This protein undergoes cation-mediated polymerization to filamentous actin; it is this form which couples with myosin to generate muscular force. We are using phosphorescence anisotropy to measure the large-scale modes of motion of the molecule and the influence of divalent cations on these motions in F-actin (filamentous). We will perform studies of the intrinsic modes of motion of the molecule and other forms of internal and segmental flexibility and the influence of cations and ATP/ADP on these motions (polymerization is mediated by ATP hydrolysis) and how these motions change in the filamentous form. The basic idea is to investigate how the modes of motion play a role in its physiological function (actomyosin force generation) and how the motion of the monomer couples/forms the basis for motion of the filament. To do this we plan on measuring tryptophan anisotropy in single trypto phan mutan ts using time correlated single photon counting methods.